Movies are, as a rule, filled with exceptional characters. A screenwriting book I once read argued that uber-successful characters are more interesting to viewers than ordinary ones. If your protagonist is a struggling actor, the book suggested, why not make him an award-winning movie star instead? Filmmakers have taken these lessons to heart, and mainstream cinema is brimming with characters — be they doctors, teachers, or Top Gun pilots — who are the best at what they do. In a sense, every movie is a superhero movie.

Not “Lady Bird,” a refreshing coming-of-age comedy from writer/director Greta Gerwig in which no one is exceptional, and as a result, the film is. Based very loosely on Gerwig’s own upbringing in Sacramento, “Lady Bird” tells the story of one ordinary girl’s senior year of high school, and her efforts to claw her way out from under the thumb of her hyper-critical mother (Laurie Metcalf) and well-meaning but ineffectual father (Tracy Letts). She dates boys, fights with her best friend, and dreams of escaping her boring hometown for a more vibrant life in the big city.

On paper, it may sound like every other teen movie this side of John Hughes, but Gerwig upends all the conventions of the genre. To wit, consider Christine’s nickname. As a symbolic rejection of her family, she calls herself Lady Bird. Instead of some punk name designed to shock her elders, it’s a name so old-fashioned that it can only be real. Similarly, the Catholic school she attends would, in a lesser film, be a source of extreme oppression, but the nuns are actually nice. When Lady Bird pastes a sign reading “Just Married to Jesus” on her teacher’s car to impress a cool friend, the elderly nun just laughs it off.

Her family is poor (they literally live on the wrong side of the tracks), and even though she has neither the grades nor the finances for it, she applies to some top college on the East Coast. Like many before her, she believes she’ll find herself in New York, far away from the grip of her family. Meanwhile, she prepares for adult life by trying to lose her virginity, dating first a sweet, fumbling theater major (Lucas Hedges), and then swerving towards a cigarette-smoking bad boy (Timothee Chamalet), who plays in a band, reads Howard Zinn, and talks about living his life on a barter system.

These details matter. For those of us roughly of Lady Bird’s generation, they strike a chord of familiarity that is instantly endearing. We need this movie. The post-war generation got their anthem in George Lucas’s “American Graffiti,’ and Generation X had “Reality Bites.” If you came of age in the late ’90s or early 2000’s (too late for grunge, and too early to work for Google), “Lady Bird” may be the first film to authentically depict the details of your adolescence. When they started smoking cloves and listening to Reel Big Fish, I knew I’d love this movie forever.

But even if you’re not in Lady Bird’s exact demographic, the film tells its universal story with such specificity that it feels fresh and new. Saoirse Ronan again, following her magnetic turn in 2015’s “Brooklyn,” quietly commands the screen in a deceptively powerful performance. She is a character who hides her best self from the world, but Ronan makes a secret connection with the audience, ensuring that we can still see it. Meanwhile, even the least sympathetic characters – notably Metcalf’s controlling mother and Chamalet’s poseur rebel – are afforded respect, and are never punished.

The approach is emblematic of Gerwig loving and generous authorial style. Looking back at her own adolescence, she avoids the tendency to either romanticize or harshly judge the transgressions of youth. In her first solo-directed film (she co-directed the indie film “Nights and Weekends” in 2008), she has delivered a spirited and empathetic work of self-reflection, and the best coming-of-age movie in years. For viewers of any generation, “Lady Bird” is a great gift.